(1) Field of the Invention
The present invention relates to an optical amplifier, an optical amplifying apparatus, an optical transmitter for use in an optical communications system, and an optical surge suppression method for the optical amplifier, which are suitable for amplifying signal light while suppressing optical surge in, for example, an optical communication system.
(2) Description of the Related Art
Recent vigorous research and development effects directed towards optical communications systems have clarified the importance of development of optical amplifiers, such as booster amplifiers, repeaters, or preamplifiers, through utilization of techniques related to optical amplifiers which use rare-earth-element-doped fibers such as erbium(Er)-doped fibers.
As a result of the emergence of optical amplifiers, a transmission system in which signal light is relayed many times through use of optical amplifiers has received attention, as it largely contributes to economizing communications systems in the multimedia age.
Optical amplifiers generally fall into two broad categories: optical fiber amplifiers and semiconductor-type amplifiers.
The optical amplifier has an optical fiber whose core portion is doped with a rare earth element such as erbium (Er). When input light and pumping light enter the optical fiber at the same time, the optical amplifier amplifies the input light through use of the energy of the pumping light. Examples of this type of optical fiber amplifier include an erbium(Er)-doped fiber optical amplifier and a praseodymium(Pr)-doped fiber optical amplifier.
In the semiconductor-type amplifier, when a semiconductor laser is excited by injection of carriers, input light is amplified through use of the resultant excitation energy. A traveling-wave semiconductor amplifier is an example of the semiconductor-type amplifier.
However, in the above-described optical amplifier, when signals are cut off, no input light is input to the optical amplifier. In a case where there is no input light and therefore the input power is low, the optical amplifier outputs light having an extremely low output intensity (indicated by M in FIG. 35) as compared to a steady-state output level.
When no input light exists in the optical amplifier, the gain of the optical amplifier recovered from a saturated level, and hence the optical amplifier is in a high-gain state.
If after completion of interruption of signal inputs there is an input of an signal light in the form of a waveform having a sharp rising edge when the optical amplifier is in such a high-gain state, the optical amplifier amplifies the rising edge of the optical signal with a high gain. As a result, a spike (optical surge) is generated in an optical output as indicated by S in FIG. 35.
If such an optical surge is generated in the optical amplifier used in the optical transmission system, it is amplified and relayed cumulatively by other relay amplifiers through the transmission line. Eventually, a terminal station receives signal light together with a much higher optical surge, thereby adversely affecting the quality of optical components at a receiving section.
Specifically, the generation of optical surges results in an increase in the intensity (optical level) of signal light input to an optical component, such as a photodiode, which serves as a light-receiving element for receiving the signal light at the receiving section. This overloads the optical component or a receiving circuit to such an extent as to exceed their tolerances. In some cases, the optical component or the receiving circuit may be damaged by the overload.